Mirrors designed for use in space and airborne high thermal energy applications (e.g. solar concentrators and laser mirrors) must not only be lightweight (to satisfy payload launch requirements), but must possess a high precision optical smoothness and flatness that is dimensionally stable over a wide range of thermal inputs.
For this purpose, the use of lightweight composites, particularly glass laminate structures employing a carbon-carbon substrate, has been proposed. One example of such a glass laminate-on-carbon-carbon mirror structure is detailed in U.S. Pat. No. 4,451,119 to Meyers et al. As described therein, because of its strength, low weight and thermal stability properties, a carbon-carbon substrate offers a particularly attractive substrate or backing material for supporting a reflective surface. However, because of its porosity and generally rough fibrous surface, a carbon-carbon substrate offers a surface of poor optical quality upon which to form a highly reflective coating. Consequently, the patentees propose the use of an intermediate bonding laminate structure of silicon carbide and glass that serves to fill in the voids in the carbon-carbon substrate and provides an adhesion layer that can be polished to optical quality for receiving a reflective coating. Unfortunately, such a structure suffers from a number of drawbacks that the use of a carbon-carbon substrate seeks to overcome.
More specifically, the choice of a carbon-carbon substrate is predicated upon its thermal stability (low coefficient of thermal expansion) and its substantial load-bearing to weight capacity. These performance advantages are diminished in the patented mirror structure because of the interposition of the glass laminate between the carbon-carbon substrate and the optical cladding layer. The glass laminate adds substantial mass, on the one hand and, most significantly, constitutes a thermal insulating barrier between the optical cladding layer and its underlying carbon-carbon support substrate. As a result, the high thermal inputs with which the mirror is intended to be used cause a substantial heating of the glass laminate structure and eventually lead to a delamination of the intermediate adhesion layers, thus effectively warping and degrading the reflective surface.